Steering control systems have traditionally relied on sensors for inputs such as hand wheel angle, driver torque, and vehicle speed. However, some parameters that affect the steering system are neither sensed nor estimated for the purposes of control due to the cost and difficulty of implementation. One example is the load at the steering system rack.
The load at the rack not only affects vehicle centric behavior, for example, free control and return, but also impacts the overall steering feel. The rack load is a highly nonlinear, highly dynamic, vehicle speed sensitive function of the rack position, thus, making it complicated to estimate. Control methods that treat the vehicle as a simple stiffness (and hence rack load as a gain times rack position) lack capturing the dynamic nature of the steering-chassis linkage.